A search for mechanisms producing circular polarization in comets

Resumen

In this work, a systematic study aimed at identifying all possible mecha- nisms producing the observed non-zero degree of circular polarization in the light scattered by comets is presented, In order to make a list of possible mechanisms, we first derived a necessary condition, which is that the symme- try of the system around the direction of the incident light must be broken somehow. Then, we made a search for the mechanisms fulflling this con- dition. The first mechanism tested was the grain alignment, that has been proposed by several authors as the cause for the presence of a certain de- gree of circular polarization in comets. To this end, we developed a Monte Carlo model of grain alignment by solar wind particles. We found that elon- gated particles do align in a few hours by this mechanism, but they do not produce any degree of circular polarization owing to the azimuthally sym- metrical configuration with respect to the direction of the incident light that results from the alignment. The second mechanism tested was the single scattering of light by optically inactive asymmetrical particles. In this case (just for two specific model shapes), we found a significant degree of circular polarization in the scattered light, comparable to, or even higher than, the observed. However, in a real cometary atmosphere there must exist a large variety of asymmetrical particles with different sizes and shapes, so that an extension of these calculations to samples of distributions of asymmetrical particles of many different sizes and shapes was also conducted. The result was negative, in the sense that for the whole sample the degree of circular po- larization tends asymptotically to zero as more and more particles are added to the system. Third, we ruled out the possibility of the optical activity of the materials forming the dust particles of the coma to be responsible for the degree of circular polarization observed in comets. The reason is that the produced results would not match some of the features we found in the observations. The fourth, and last, mechanism we tried was the presence of multiple scattering in an azimuthally asymmetrical scenario around the direction of the incoming light. To accomplish this, a Monte Carlo model of radiative transfer in cometary comae was developed. The model is capable of giving the flux, and the degree of linear and circular polarization of light emerging from dusty cometary environments, under conditions of either sin- gle or multiple scattering. After trying several scenarios corresponding to dferent physical parameters of the cometary atmosphere, and different ob- servational conditions, we concluded that the multiple scattering processes are the only mechanism that might give rise to the observed degree of circular polarization by itself.